Long Term Evolution (LTE) specifies a system which enables a mobile station (UE) to perform voice over LTE (VoLTE) over an IP-Connectivity Access Network (IP-CAN) by establishing IP-CAN Bearer (SIP bearer) on the IP-CAN. Here, the IP-CAN is an IP access network providing a function of transporting Session Initiation Protocol (SIP) and audio media, and the SIP bearer is a logical communication channel for SIP and audio media (see, for example, Non-patent document 1).
Further, it is specified that if the IP-CAN Bearer is removed for some reason (e.g., handover of a mobile station to UTRAN/GERAN, network-side overloading, and the like), the mobile station executes a procedure for reconnection to the IP-CAN (e.g., Non-patent document 2).
FIG. 1 shows a communication sequence in a case where IP-CAN Bearer established by a mobile station via E-UTRAN (LTE) is removed due to handover (transition) to UTRAN/GERAN which is a different radio access network. As shown in FIG. 1, when the mobile station performs handover to UTRAN/GERAN (S10), the IP-CAN Bearer is removed (removal) (S20). Then, after executing the handover, the mobile station executes a procedure for reconnection to the IP-CAN (S30).